1. Field of the Invention
The present invention relates to an interrogator for a RFID tag communication system for performing reading or writing of information with respect to a RFID tag capable of wireless communication of information with the outside.
2. Description of the Related Art
A RFID (Radio Frequency Identification) system is known in which reading/writing of RFID tag information is performed by transmitting a query to and receiving a reply from a small-sized RFID tag as a transponder in a non-contact manner by means of a reader/writer as an interrogator.
For example, a RFID circuit element provided in a label-like RFID tag is equipped with an IC circuit part for storing predetermined RFID tag information, and an antenna connected to this IC circuit part to transmit/receive information. Since the reader/writer can make access (reading/writing of information) to the IC circuit part even in such a case where the RFID tag is soiled or the RFID tag is arranged at a position where it cannot be seen, the practical use of RFID systems is being anticipated for a variety of fields such as merchandise management and inspection processes.
In the related art, there is a technique according to which when making access to RFID tag information of the IC circuit part of the RFID circuit element as described above, by taking advantage of the fact that a RFID circuit element located closer to the device-side antenna can be accessed by means of a smaller output, a signal of a given output value is transmitted and a group of tags responding to this signal is recognized, and thereafter, these tags are put to sleep and a signal of a slightly larger output value is transmitted, and a group of tags responding to this signal is recognized as a group of tags more distant than the above-mentioned group of tags that have been put to sleep, and this process is repeated to thereby recognize the distributed positions of all the tags with respect to a plurality of split areas (see, for example, Patent Document 1).
Patent Publication 1: JP,A, 11-344562 (paragraphs 0019 to 0032, FIGS. 3 and 4)
In the above-mentioned related art, while gradually increasing the output value of a transmission signal, groups of tags that have responded within a substantially annular area corresponding to the output value are sequentially recognized, and this process is repeated to thereby recognize the distributed positions of all the tags with respect to a plurality of split substantially annular areas. At this time, the directivity of the interrogator-side antenna is not particularly changed; transmission/reception of a signal is performed with broad directivity both in the case where signal transmission/reception is performed with respect to a RFID circuit element located at a relatively short distance and the case where transmission/reception is performed with respect to a RFID circuit element located at a relatively long distance.
Since groups of tags located within the substantially annular areas are integrally recognized, to make access to these tags individually, it is necessary to perform a separate search by sequentially specifying a part of identification information of each of the tags, so the required identification time greatly increases as the number of tags becomes larger. As a result, when RFID circuit elements are distributed over a broad area, a huge amount of time is required for making access to each of the RFID circuit elements, making it impossible to perform transmission/reception of RFID tag information efficiently in a short period of time.